1. Field of and the Invention
The present invention relates to magnetic read write heads, and in particular to a magnetic head for writing a plurality of evenly spaced, high tolerance servo information tracks on the full width of a magnetic medium, such as a magnetic tape, in a single pass. The invention also relates to a process for making such magnetic heads.
2. The Prior Art
It has long been an important objective of designers in the magnetic storage technology to provide magnetic media having data stored thereon in high track densities with high lineal densities. In seeking this objective, it has been recognized that higher density recording can be more easily achieved if a servo signal, such as a track location indicating signal, is superposed on the magnetic media with the data signal. One system for accomplishing this utilizes a magnetic medium comprising a dual layer magnetic coating on a support surface. The lower of the two magnetic layers has a higher retentivity and coercivity than the upper magnetic layer.
Servo signals are first written in the lower layer to form parallel, adjacent servo tracks. Servo signals are written in adjacent servo tracks at different frequencies. Data signals are subsequently written in the upper layer along data tracks that are defined through reference to the servo information in the lower layer. For example, each data track may be defined as being superimposed over 1/2 of an odd numbered servo track and 1/2 of an even numbered servo track. By providing a magnetic medium wherein the lower servo level has a higher coercivity than the upper data level, the servo signals are allowed to remain undisturbed by subsequent writing and rewriting of data signals of lower magnetic field strength in the upper layer.
Such a track-following servo system permits accurate alignment of the data transducer or data read and write head with a recording track. The positioning is accomplished by simultaneously reading a data track and the servo tracks on either side thereof, and then filtering the data from the servo information. The amplitude of the high frequency and lower frequency servo information from adjacent servo tracks is then compared to develop a position error signal that indicates whether the servo read head is correctly positioned between the two servo tracks, or whether it is located more above one of the tracks than the other. The error signal is then supplied to an actuator, which responds by adjusting the position of the data transducer.
The prior applications of this technology have typically been in connection with magnetic media comprising magnetic disks. A significant problem has been encountered in attempting to place these types of parallel servo tracks on magnetic disk media. This problem comprises the technological difficulty of maintaining the linear density of the servo information in the track as the track progresses toward the center of the disk. For example, at a constant rotation rate of the magnetic disk, the velocity of a track close to the center of the disk is much slower than the velocity of a track close to the outside edge of the disk. Accordingly, writing information onto the disk at a constant writing rate will produce a linear density of information that increases in density as the proximity of the track to the center of the disk increases.
Two primary options are available for maintaining a constant linear density on the various tracks of a disk. In the first option, if the rotational speed of the disk is held constant, the frequency of the servo information being written onto the disk must be decreased as the distance from the track to the center of the disk decreases. In the second option, if the frequency of the servo information is held constant, then the rotational speed of the disk must be increased as the distance from the track to the center of the disk decreases. Of course, a third option would be to provide a combination of increasing the speed of the disk, as well as reducing the frequency of the servo data as the track distance from the disk center decreases.
Because of the above-identified problems, it has been necessary in prior art systems to write the servo data tracks into the magnetic media one track at a time. This process is time consuming, and suffers from a relatively high degree of inaccuracy in positioning the servo tracks in consistently close proximity to one another. For example, after a first servo track is written onto the magnetic media, the position of the servo writing head is adjusted to place the next track adjacent to the first track. It is very difficult to accomplish this mechanical adjustment of the head to provide a very high accuracy in the track alignment. It is even more difficult to maintain this alignment throughout the entire trip around the magnetic disk, due to factors such as mechanical play, discontinuities in the disk surface, and changes in physical properties of the disk caused by changing environmental conditions, such as changes in temperature.
Many of the problems associated with use of magnetic disks can be overcome through the use of a magnetic tape as the recording medium. For example, since the tape speed past a stationary head is constant across the entire width of the tape, a multi-track write head could be utilized to write servo tracks onto the tape. This would accomplish the accurate alignment of the tracks with respect to one another since they would be simultaneously written by the fixed writing gaps of the multi-track head. Magnetic tapes that have substantially consistent levels of coercivity are well-known and commonly used in data storage and handling applications. The use of two data storage layers for recording signals has been described for application in magnetic tapes in references such as U.S. Pat. Nos. 3,328,195 to May, and 4,075,384 to Suzuki et al., each of these references being incorporated herein by reference. Multiple track heads are also known in the art. Nevertheless, an accurate and efficient system and method for writing the servo information onto such a magnetic recording medium in a single pass of the writing head has not been described in the prior art.
Another problem experienced in prior art systems has involved separating the servo information from the other data detected by a conventional read head. For example, when a read head is used with a multi-layer magnetic medium having both the servo information and the data simultaneously detected by the read head, a scheme must be provided for separating the servo information from the data. This separation is generally accomplished by application of filtering techniques. These techniques typically require circuitry dedicated specifically for this separation function, increasing the cost and complexity of the read systems. Although it would be very desirable, the prior art has not described a system and method for writing the servo information onto the magnetic medium in a single pass, as described above, and in an orientation such that properly oriented read heads may subsequently read either the servo information or the data only.
In view of the above, it would be an important improvement in the technology to provide an apparatus and method for writing a plurality of servo tracks onto a magnetic medium with a high degree of accuracy to enable subsequent track following systems to write signals on the magnetic medium with high track densities and with high lineal densities. It would be a further improvement in the technology to provide such a system and method that would accomplish this writing step in a single pass of the magnetic medium under the write head. It would be a still further improvement to provide such a system and method that would write the servo information onto the magnetic medium at an orientation such that properly oriented read heads may subsequently read either the servo information or the data only. It would also be an important improvement in the technology to define a low cost and efficient method for manufacturing such a servo writing system.